5K PR during training brick? Huh?

I did a brick today with a 15 mile bike (not terribly fast- just over 15 MPH, but doing intervals where I rode fast(ish) for 2 miles, then took it easy), then planned an easy 3 mile run.  I did a tri last month, and my legs felt like lead after the bike, so I've been trying to add a brick a week to get in better shape.  I did a quick transition (3-4 minutes), and then started running.  My legs felt tired but I was able to clop along, and the music was good, and next thing I knew I was flying back to my car, with a 5K PR of 26:12!  WTF??? I think the first lesson is that I should back off on the last mile of the bike.  But I have a few other questions:

1)  Why on earth would I go faster after 15 miles of biking than I do on fresh legs?

2)  Should I be warming up more before my 5K races?

3)  Was going that fast during a training run stupid (my heart rate monitor was conveniently forgotten at home)?

4) How do I make it happen again on race day ?

Thanks to anyone who might have ideas!